1. Field of the Invention
The present invention relates generally to rotary-type drag bits used in drilling subterranean formations and, more particularly, to such drill bits employing chip breakers to facilitate the break up of formation chips generated during drilling, resulting in more effective removal of the chips from around the drill bit with drilling fluid.
2. State of the Art
Fixed-cutter rotary drag bits have been employed in subterranean drilling for many decades with various sizes, shapes and patterns of natural and synthetic diamonds used on drag bit crowns as cutting elements. Rotary drag-type drill bits are typically comprised of a bit body having a shank for connection to a drill string and encompassing an inner channel for supplying drilling fluid to the face of the bit through nozzles or other apertures. Drag bits may be cast and/or machined from metal, typically steel, or may be formed of a powder metal (typically tungsten carbide (WC)) infiltrated at high temperatures with a liquified (typically copper-based) binder material to form a matrix. Such bits may also be formed with layered-manufacturing technology, as disclosed in U.S. Pat. No. 5,433,280, assigned to the assignee of the present invention and incorporated herein by this reference.
The bit body typically carries a plurality of cutting elements mounted directly on the face of the bit body or on carrier elements adjacent fluid courses to allow cuttings (i.e., formation chips) generated during drilling to flow from the cutting elements to and through junk slots on the gage of the bit and to the bore hole annulus above the bit. Cutting elements may be secured to the bit by preliminary bonding to a carrier element, such as a stud, post, or cylinder, which in turn is inserted into a pocket, socket, recess or other aperture in the face of the bit and mechanically or metallurgically secured thereto.
One type of drag bit includes polycrystalline diamond compact (PDC) cutters typically comprised of a large diamond table (usually of circular, semi circular or tombstone shape) which presents a generally planar cutting face. A cutting edge (sometimes chamfered or beveled) is formed on one side of the cutting face which, during boring, is at least partially embedded into the formation so that the formation is received against at least a portion of the cutting face. As the bit rotates, the cutting face moves against the formation and shavings of formation material are sheared off and ride up the surface of the cutter face. In brittle materials the shavings easily separate from the cutter face and break down into small particles that are transported out of the bore hole via circulating drilling fluid. Another shaving then begins to form in the vicinity of the cutting edge, slides up the face of the cutting surface, and breaks off in a similar fashion. Such action occurring at each cutting element on the bit removes formation material over the entire face of the bit, and so causes the bore hole to become progressively deeper.
However, in formations that behave more plastically, such as highly pressurized deep shales, mudstones, and siltstones, the formation shavings have a marked tendency to stay intact and adhere to the cutting face of the cutting element.
When these formation shavings adhere to the cutting face and do not break into smaller pieces, the shavings tend to collect and to build up as a mass of cuttings ahead of the PDC cutting elements and eventually clog the entire open bit space with drilled-up material. Once the bit is clogged with drilled-up material, the bit ceases to drill effectively.
Undesired accumulation of shavings, or cuttings, from subterranean formations being drilled by drag bit PDC cutting elements has long been recognized as a problem in the subterranean drilling art, particularly in formations of highly pressurized shale. A number of different approaches have been attempted to facilitate removal of formation cuttings from the cutting face of PDC cutting elements. For example, U.S. Pat. No. 5,582,258 to Tibbitts et al., assigned to the assignee of the present invention and herein incorporated by this reference, includes a chip breaker formed adjacent the cutting edge of the cutting elements to impart strain to a formation shaving by bending and/or twisting the shaving and thereby increasing the likelihood that the chip will break away from the face of the blade portion of the bit. Other approaches to solving the problem of formation chip removal include U.S. Pat. No. 4,606,418 to Thompson, which discloses cutting elements having an aperture in the center thereof which feeds drilling fluid from the interior of the drill bit onto the cutting face to cool the diamond table and to remove formation cuttings. U.S. Pat. No. 4,852,671 to Southland discloses a diamond cutting element which has a passage extending from the support structure of the cutting element to the extreme outermost portion of the cutting element, which is notched in the area in which it engages the formation being cut so that drilling fluid from a plenum on the interior of the bit can be fed through the support structure and to the edge of the cutting element immediately adjacent the formation. U.S. Pat. No. 4,984,642 to Renard et al. discloses a cutting element having a ridged or grooved cutting face on the diamond table to promote the break up of formation chips, or, in the case of a machine tool, the break up of chips of material being machined, and enhance their removal from the cutting face. The irregular topography of the cutting face assists in preventing balling or clogging of the bit by reducing the effective surface or contact area of the cutting face, which also reduces the pressure differential of the formation chips being cut. U.S. Pat. No. 5,172,778 to Tibbitts et al., assigned to the assignee of the present application, employs ridged, grooved, stair-stepped, scalloped, waved and other alternative non planar cutting surface topographies to permit and promote the access of fluid in the bore hole to the area on the cutting face of the cutting element immediately adjacent to and above the point of engagement with the formation. Such a non planar cutting surface helps to equalize differential pressure across the formation chip being cut and thus reduce the shear force which opposes chip movement across the cutting surface. U.S. Pat. No. 4,883,132 to Tibbitts, assigned to the assignee of the present application, discloses a novel drill bit design providing large cavities between the face of the bit and the cutting elements engaging the formation. Formation cuttings entering the cavity area are thus unsupported and more likely to break off for transport up the bore hole. In addition, clearing of the chips is facilitated by nozzles aimed from behind the cutting elements (taken in the direction of bit rotation) so that the chips are impacted in a forward direction to break off immediately after being cut from the formation. U.S. Pat. No. 4,913,244 to Trujillo, assigned to the assignee of the present invention, discloses bits which employ large cutters having associated therewith directed jets of drilling fluid emanating from specifically oriented nozzles placed in the face of the bit in front of the cutting elements. The jet of drilling fluid is oriented so that the jet impacts between the cutting face of the cutting element and a formation chip as it is moving along the cutting face to peel the chip away from the cutting element and toward the gage of the bit. Likewise, GB 2,085,945 to Jurgens provides nozzles that direct drilling fluid toward the cutting elements to flush away cuttings generated by the cutting elements. U.S. Pat. No. 5,447,208 to Lund et al., assigned to the assignee of the present invention, discloses a superhard cutting element having a polished, low-friction, substantially planar cutting face to reduce chip adhesion across the cutting face. Finally, U.S. Pat. No. 5,115,873 to Pastusek, assigned to the assignee of the present application, discloses yet another manner in which formation cuttings can be removed from a cutting element by use of a structure adjacent and/or incorporated with the face of the cutting element to direct drilling fluid to the face of the cutting element and behind the formation chip as it comes off the formation.
It will be appreciated by those skilled in the art that the foregoing approaches require significant modification to the cutting elements themselves, to the structure carrying the cutting elements on the bit face, and/or to the bit itself Thus, the foregoing approaches to the problem require significant expenditures which substantially raise the price of the drill bit. In addition, due to required cutter placement on certain styles and sizes of bits for efficient and effective cutting, many of the prior art hydraulic chip removal arrangements are unsuitable for general application. Accordingly, it would be desirable to provide the industry with a solution to the breakdown and dispersion of large chips, or shavings, which solution could be economically effected on any fixed-cutter drill bit regardless of size or style, and regardless of the type of formation which might be expected to be encountered by the drill bit.